Arrange for command to be executed whenever command name
is modified in one of the ways given by the list ops. Name will be
resolved using the usual namespace resolution rules used by
procedures. If the command does not exist, an error will be thrown.

Ops indicates which operations are of interest, and is a list of
one or more of the following items:

Invoke command when the command is deleted. Commands can be
deleted explicitly by using the rename command to rename the
command to an empty string. Commands are also deleted when the
interpreter is deleted, but traces will not be invoked because there is no
interpreter in which to execute them.

When the trace triggers, depending on the operations being traced, a
number of arguments are appended to command so that the actual
command is as follows:

command oldName newName op

OldName and newName give the traced command's current
(old) name, and the name to which it is being renamed (the empty
string if this is a 'delete' operation).
Op indicates what operation is being performed on the
command, and is one of rename or delete as
defined above. The trace operation cannot be used to stop a command
from being deleted. Tcl will always remove the command once the trace
is complete. Recursive renaming or deleting will not cause further traces
of the same type to be evaluated, so a delete trace which itself
deletes the command, or a rename trace which itself renames the
command will not cause further trace evaluations to occur.
Both oldName and newName are fully qualified with any namespace(s)
in which they appear.

Arrange for command to be executed whenever command name
is executed, with traces occurring at the points indicated by the list
ops. Name will be
resolved using the usual namespace resolution rules used by
procedures. If the command does not exist, an error will be thrown.

Ops indicates which operations are of interest, and is a list of
one or more of the following items:

Invoke command for every Tcl command which is executed
inside the procedure name, just before the actual execution
takes place. For example if we have 'proc foo {} { puts "hello" }',
then an enterstep trace would be
invoked just before puts "hello" is executed.
Setting an enterstep trace on a command
will not result in an error and is simply ignored.

Invoke command for every Tcl command which is executed
inside the procedure name, just after the actual execution
takes place.
Setting a leavestep trace on a command
will not result in an error and is simply ignored.

When the trace triggers, depending on the operations being traced, a
number of arguments are appended to command so that the actual
command is as follows:

For enter and enterstep operations:

command command-string op

Command-string gives the complete current command being
executed (the traced command for a enter operation, an
arbitrary command for a enterstep operation), including
all arguments in their fully expanded form.
Op indicates what operation is being performed on the
command execution, and is one of enter or enterstep as
defined above. The trace operation can be used to stop the
command from executing, by deleting the command in question. Of
course when the command is subsequently executed, an 'invalid command'
error will occur.

For leave and leavestep operations:

command command-string code result op

Command-string gives the complete current command being
executed (the traced command for a enter operation, an
arbitrary command for a enterstep operation), including
all arguments in their fully expanded form.
Code gives the result code of that execution, and result
the result string.
Op indicates what operation is being performed on the
command execution, and is one of leave or leavestep as
defined above.
Note that the creation of many enterstep or
leavestep traces can lead to unintuitive results, since the
invoked commands from one trace can themselves lead to further
command invocations for other traces.

Command executes in the same context as the code that invoked
the traced operation: thus the command, if invoked from a procedure,
will have access to the same local variables as code in the procedure.
This context may be different than the context in which the trace was
created. If command invokes a procedure (which it normally does)
then the procedure will have to use upvar or uplevel commands if it wishes
to access the local variables of the code which invoked the trace operation.

While command is executing during an execution trace, traces
on name are temporarily disabled. This allows the command
to execute name in its body without invoking any other traces again.
If an error occurs while executing the command body, then the
command name as a whole will return that same error.

When multiple traces are set on name, then for enter
and enterstep operations, the traced commands are invoked
in the reverse order of how the traces were originally created;
and for leave and leavestep operations, the traced
commands are invoked in the original order of creation.

The behavior of execution traces is currently undefined for a command
name imported into another namespace.

Arrange for command to be executed whenever variable name
is accessed in one of the ways given by the list ops. Name may
refer to a normal variable, an element of an array, or to an array
as a whole (i.e. name may be just the name of an array, with no
parenthesized index). If name refers to a whole array, then
command is invoked whenever any element of the array is
manipulated. If the variable does not exist, it will be created but
will not be given a value, so it will be visible to namespace which
queries, but not to info exists queries.

Ops indicates which operations are of interest, and is a list of
one or more of the following items:

Invoke command whenever the variable is accessed or modified via
the array command, provided that name is not a scalar
variable at the time that the array command is invoked. If
name is a scalar variable, the access via the array
command will not trigger the trace.

Invoke command whenever the variable is unset. Variables
can be unset explicitly with the unset command, or
implicitly when procedures return (all of their local variables
are unset). Variables are also unset when interpreters are
deleted, but traces will not be invoked because there is no
interpreter in which to execute them.

When the trace triggers, three arguments are appended to
command so that the actual command is as follows:

command name1 name2 op

Name1 and name2 give the name(s) for the variable
being accessed: if the variable is a scalar then name1
gives the variable's name and name2 is an empty string;
if the variable is an array element then name1 gives the
name of the array and name2 gives the index into the array;
if an entire array is being deleted and the trace was registered
on the overall array, rather than a single element, then name1
gives the array name and name2 is an empty string.
Name1 and name2 are not necessarily the same as the
name used in the trace variable command: the upvar
command allows a procedure to reference a variable under a
different name.
Op indicates what operation is being performed on the
variable, and is one of read, write, or unset as
defined above.

Command executes in the same context as the code that invoked
the traced operation: if the variable was accessed as part of a Tcl
procedure, then command will have access to the same local
variables as code in the procedure. This context may be different
than the context in which the trace was created. If command
invokes a procedure (which it normally does) then the procedure will
have to use upvar or uplevel if it wishes to access the
traced variable. Note also that name1 may not necessarily be
the same as the name used to set the trace on the variable;
differences can occur if the access is made through a variable defined
with the upvar command.

For read and write traces, command can modify the variable to
affect the result of the traced operation. If command modifies
the value of a variable during a read or write trace, then the new
value will be returned as the result of the traced operation. The
return value from command is ignored except that if it returns
an error of any sort then the traced operation also returns an error
with the same error message returned by the trace command (this
mechanism can be used to implement read-only variables, for example).
For write traces, command is invoked after the variable's value
has been changed; it can write a new value into the variable to
override the original value specified in the write operation. To
implement read-only variables, command will have to restore the
old value of the variable.

While command is executing during a read or write trace, traces
on the variable are temporarily disabled. This means that reads and
writes invoked by command will occur directly, without invoking
command (or any other traces) again. However, if command
unsets the variable then unset traces will be invoked.

When an unset trace is invoked, the variable has already been deleted:
it will appear to be undefined with no traces. If an unset occurs
because of a procedure return, then the trace will be invoked in the
variable context of the procedure being returned to: the stack frame
of the returning procedure will no longer exist. Traces are not
disabled during unset traces, so if an unset trace command creates a
new trace and accesses the variable, the trace will be invoked. Any
errors in unset traces are ignored.

If there are multiple traces on a variable they are invoked in order
of creation, most-recent first. If one trace returns an error, then
no further traces are invoked for the variable. If an array element
has a trace set, and there is also a trace set on the array as a
whole, the trace on the overall array is invoked before the one on the
element.

Once created, the trace remains in effect either until the trace is
removed with the trace remove variable command described below,
until the variable is unset, or until the interpreter is deleted.
Unsetting an element of array will remove any traces on that element,
but will not remove traces on the overall array.

If there is a trace set on command name with the operations and
command given by opList and command, then the trace is
removed, so that command will never again be invoked. Returns
an empty string. If name doesn't exist, the command will throw
an error.

If there is a trace set on command name with the operations and
command given by opList and command, then the trace is
removed, so that command will never again be invoked. Returns
an empty string. If name doesn't exist, the command will throw
an error.

Returns a list containing one element for each trace currently set on
command name. Each element of the list is itself a list
containing two elements, which are the opList and command
associated with the trace. If name doesn't have any traces set,
then the result of the command will be an empty string. If name
doesn't exist, the command will throw an error.

Returns a list containing one element for each trace currently set on
command name. Each element of the list is itself a list
containing two elements, which are the opList and command
associated with the trace. If name doesn't have any traces set,
then the result of the command will be an empty string. If name
doesn't exist, the command will throw an error.

Returns a list containing one element for each trace currently set on
variable name. Each element of the list is itself a list
containing two elements, which are the opList and command
associated with the trace. If name doesn't exist or doesn't
have any traces set, then the result of the command will be an empty
string.

These subcommands are deprecated and will likely be removed in a
future version of Tcl. They use an older syntax in which array,
read, write, unset are replaced by a, r,
w and u respectively, and the ops argument is not a
list, but simply a string concatenation of the operations, such as
rwua.